Cardiovascular disease is a primary health threat. Therefore, it is essential to detect cardiovascular disease in its early stages. One manner to detect cardiovascular disease is through the use of magnetic resonance angiography (MRA). The primary goal of MRA is to non-invasively acquire necessary clinical information via image data to diagnose and plan treatment for various cardiovascular problems. Unlike the current standard of treatment, X-ray angiography, MRA does not require insertion of a catheter for introduction of the contrast agent. Additionally, X-ray angiography (XRA) contrast agents are nephrotoxic, whereas MRA contrast agents are typically much better tolerated by the body. Although MRA is only beginning to emerge into mainstream clinical practice, it has the potential to become the primary modality to image cardiovascular tissues in the future.
In MRA, the first generation of contrast agents introduced into the clinical market were known as ECF agents. These agents rapidly leave the bloodstream and ‘leak’ into the fluids in the body, reducing contrast between the blood pool and the rest of the body quickly. Intravascular agents, through a variety of mechanisms, extend the persistance of the contrast agent in the blood pool, allowing for longer image acquisition periods. Until intravascular agents were introduced, MRA image acquisition was typically limited to the first pass of the contrast bolus, much like XRA, due to the extensive leakage of the contrast agent into the extracellular fluid. Intravascular agents remain in the blood pool much longer, allowing for longer MR scan times, greater potential resolution, and less reliance on timing of the introduction of the contrast bolus, etc. This longer persistance in the blood, however, introduces a problem in visualization. Because of simultaneous enhancement of both the arterial and venous blood pools, the vessels obscure each other when using projection methods (such as maximum intensity projection (MIP)), and can potentially confuse the reader even when viewing the source data.
Prior methods for manipulating the visualization of MRA data include segmentation methodologies and Digital Subtraction Angiography (DSA). Each of these methods, however, disadvantageously remove a significant amount of potentially useful information and/or have reliability issues. Additionally, reliance on capturing a contrast bolus delivery (for DSA) can be difficult, or almost impossible (as in carotid imaging). Additionally, DSA has been presented in the past as a method of removing background information, not as a technique for removal or reduction of other vascular structures. These vascular structures, however, also contain potentially relevant diagnostic information. Computer segmentation methods alone allow for the loss of supporting information, such as anatomic landmarks from surrounding anatomy, and exclusion of potentially clinically significant information.
Previous work in the area of angiographic image processing tends to relate to the removal of background information to enhance vessel visibility without removing the high intensity signal that represents the blood pool. U.S. Pat. No. 5,297,551 discloses a method of intensity manipulation. However, a method such as this does not effectively differentiate between the arterial and venous blood pools, which in this case is very desirable.
This task of differentiating between the arterial and venous blood pools is effectively independent of enhancing visibility by manipulating the background regions. Because no contrast agents until the present time would remain in the blood long enough to effectively enhance both blood pools at the same level simultaneously, no previous work is known to have been done to effectively overcome this effect.
U.S. Pat. No. 6,073,042 to Simonetti shows a method of displaying three-dimensional MRA images in which arteries can be distinguished from veins. The Simonetti method, however, requires acquiring multiple image series as a function of time and requires many calculations to find curves that simulate the change of voxel intensity as a function of time. As is the case with most MRA methods, image sets are closely temporally spaced (i.e. short TR) in order to accurately represent dynamic phenomena. Unfortunately, this necessarily limits the resolution of the resultant images compared with images required over longer time periods. There remains a need for MRI techniques that faithfully convey physiological phenomena in images with high spatial resolution. In the case of MRA, there remains a need for techniques which present high resolution images of the vasculature and indicate whether those vessels are arteries or veins.